Now, I’ll try to sum everything up. First of all, the single Radeon HD 5870 does not depend as much on the CPU as it is supposed to. According to my tests (the left part of the diagrams), the Radeon HD 5870 is quite satisfied with an overclocked dual-core CPU manufactured two years ago. And you can even leave the CPU at its default frequency at the high-quality settings and 1920x1200. The only exceptions are Left 4 Dead and Warhammer 40000: Dawn of War II. The former game is not a problem for modern top-end Radeons while the latter, on the contrary, calls for a quad-core CPU, preferably from Intel.
The Radeon HD 5870 CrossFireX configuration is more CPU-dependent than the single such card. But it is not possible to say what CPU is sufficient for this graphics subsystem as this depends on the particular game, settings and resolutions. For example, at 1920x1200 with 4(8)x FSAA and 16x AF an Intel Core i7 at 2.67GHz is quite enough for S.T.A.L.K.E.R.: Clear Sky, Far Cry 2, BattleForge, Tom Clancy's H.A.W.X. and Resident Evil 5 as well as for Unigine Heaven Demo. In the other games and semi-synthetic benchmarks the Radeon HD 5870 tandem calls for a faster CPU. The question is whether you really need those 150-200 frames per second.
I also want to say a few words about the platform with the dual-core CPU as compared with the quad-core CPU. Winding up this test session I carried out an interesting experiment. I connected two system cases with a Radeon HD 5870, one with an Intel Core 2 Duo at 4.1GHz and another with an Intel Core i7 at 4.1GHz, simultaneously to the same monitor. Then I selected the most CPU-indifferent resolution of 1920x1200 with FSAA and AF and launched a few games. I switched between the two systems by choosing the input on the monitor (analog/DVI-I) and used two mice and two keyboards.
Using this setup, I could quickly switch from the system with a dual-core CPU to the system with a quad-core CPU and feel the difference between them in the same parts of five games. Excepting S.T.A.L.K.E.R.: Clear Sky, I easily felt the difference such games as Tom Clancy's H.A.W.X., Resident Evil 5, Call of Duty 5: World at War and Crysis. The gameplay was much smoother on the quad-core CPU, without occasional jerks as on the Core 2 Duo. Game levels were loaded faster, too. So, the numbers don’t always provide the full picture, yet I hope that this test session will help you decide what CPU you need for one or two Radeon HD 5870 cards.